The present invention relates to a system for controlling transmission ratio of a continuously variable belt-drive automatic transmission (CVT) for a motor vehicle having an antilock brake system.
A known control system for a continuously variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to the servo devices, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
An electronic control system is provided for controlling the hydraulic circuit corresponding to the driving conditions of the motor vehicle. An optimum desired transmission ratio is determined in accordance with vehicle speed and throttle valve opening degree to obtain the transmission ratio. Japanese Patent Application Laid-Open 64-52535 submitted by the applicant discloses such a system.
The antilock brake system (ABS) is provided for preventing the wheels from completely locking at rapid braking or at braking on slippery surfaces such as a snowy road, thereby ensuring directional stability and steering control of the vehicle during the braking operation and improving safety of the vehicle.
The ABS operates in such a manner as to intermittently brake the wheels. Consequently, the rotating speed of the wheels, that is the vehicle speed measured by rotational speed of the output shaft of the transmission changes at a short interval, and hence the transmission ratio fluctuates.
Accordingly, when the ABS is in operation, the transmission is not accurately controlled corresponding to the vehicle speed, so that the downshifting of the transmission delays. On the other hand, the transmission must be downshifted to a maximum transmission ratio (low gear) when the vehicle is stopped. However, the transmission can not be completely downshifted to the maximum transmission ratio because of the delay. Therefore a sufficient acceleration can not be obtained at restarting of the vehicle.